The X-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical X-ray tubes are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor comprising a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the X-ray tube that contains the rotor. The rotor of the rotating anode assembly is driven by the stator which surrounds the rotor of the anode assembly. The X-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
One of the factors that tends to degrade X-ray, particularly computerized tomography (CT), images is the production of x-radiation at points in the X-ray tube other than the focal spot. Most of this off-focal radiation is caused by electrons that have scattered from the focal spot to hit other points of the target and the tube. In CT, this off-focal radiation creates image artifacts, particularly near the bone bran interface in head scans, where it can obscure trauma injuries or can appear as false positive medical problems. Current tube designs with software correction algorithms are marginal. New CT products being developed are more sensitive to off focal radiation since they use wider X-ray beams to accommodate multi-slice detectors and shorter geometries. Of particular importance is the need for a uniform X-ray beam in the Z-axis on the detector. Off-focal radiation creates an additional non-uniformity that requires wider collimation and, hence, increased patient X-ray dose in order to avoid the nonuniformity. The increased nonuniformity also increases the need for sophisticated software corrections and collimation apparatus.
It would be desirable, then, to have an X-ray tube structure which reduces off-focal radiation.